1. Technical Field of the Invention
The present invention relates generally to a spark plug which may be employed in automotive vehicles, gas feed pumps and cogeneration systems, and more particularly to a spark plug designed to enhance the mechanical strength of a joint of a noble metal member made of, for example, an Ir (Iridium) alloy to a ground electrode.
2. Background Art
Japanese Patent First Publication Nos. 11-354251 and 2002-93547 propose a spark plug of the above type which consists essentially of a metal shell, a center electrode retained in the metal shell in electrical insulation therefrom, and a Ni-based alloy made ground electrode. The ground electrode has joined thereon a noble metal chip which is made of Ir alloy and faces the center electrode through the spark gap.
The noble metal chip is laser-welded to the ground electrode, thereby forming wedge-shaped fused portions (also called weld nuggets) in which materials of the noble metal chip and the ground electrode are melted together to produce mechanical joints of the noble metal chip to the ground electrode.
Use of the laser welding to form the fused portions between the ground electrode and the noble metal chip results in a decrease in thermal stress airings from a difference in coefficient of linear expansion between the noble metal chip and the ground electrode.
Japanese Patent First Publication No. 11-354251 teaches a joining method of placing the noble metal chip on an area of the ground electrode facing the center electrode and emitting laser beams to the surface of the noble metal chip to form the wedge-shaped fused portions which taper from the outer surface of the noble metal chip to inside the ground electrode.
Japanese Patent First Publication No. 2002-93547 teaches another joining method of forming a recess in an area of the ground electrode facing the center electrode, fitting the noble metal chip in the recess, and emitting laser beams to the outer surface of the ground electrode to form the wedge-shaped fused portions which taper from the outer surface of the ground electrode to inside the recess. This structure is effective in improving the reliability of a joint of the noble metal chip to the ground electrode.
In recent years, higher engine output and lower fuel consumption and exhaust emissions requirements have been increased, thus resulting in elevated temperatures of a combustion atmosphere in the engines.
The inventor of this application has researched the strength of the joint between the noble metal chip and the ground electrode of the spark plug of the type, as taught in Japanese Patent First Publication No. 2002-93547, and found that when the ground electrode which will usually be the highest in temperature of the spark plug during combustion in the engine is exposed to intense heat, it may result in cracks in the fused portions which, in the worst case, causes the noble metal chip to drop out of the ground electrode, thus leasing to the misfires.